1. Field of the Invention
The present invention relates to halftone printing in laser printers and more particularly to an improved modulation structure and method for generating halftones dots.
2. Description of the Prior Art
In addition to improvements in the chemistry and physics of drums and toners, a critical factor in improving the quality of the output of laser printers is to improve the imaging capability by techniques to control the laser beam in the printer engine.
Early laser printers can only turn entire pixels completely on or completely off. To print contone images, halftoning techniques have to be applied. A common technique of halftoning is the use of halftone screening with clustered dot spot functions. The amount of gray gradation obtained through said technique depends on the size of the halftone screen. More gray values can be presented by increasing the size of the halftone screen. However, as the screen is made larger, the underlying cluster dot pattern becomes more noticeable and more objectionable to the eye.
Later laser printers allowed the control of the laser beam to be finer than complete pixels. This property was exploited in two directions. The first direction was the improvement in the number of gray levels one can obtain with clustered dot halftoning of contone images. The second direction was to exploit the more precise control for the resolution enhancement of text and graphics through anti-aliasing techniques.
The aforesaid methods require precise control of the size of a subpixel (as determined by the laser beam width) as well as the position of the subpixel within the pixel.
Existing digital techniques for modulating the laser beam permit such size and position control when the size and position increments are relatively large. Previously known analog modulation techniques allow precise control of the dot size, but not position control.
Conventional laser printers are bi-level devices. The laser beam is turned on or off only at pixel boundaries. For a halftone cell consisting of n pixels, n+1 grays are possible. For example, for an 8 pixel cell, there are 9 possible gray levels. One possible sequence of turning on the pixels known as the "spot function" is shown in FIGS. 1 through 9. The nine possible gray levels are generated by clocking through the pixel data in conventional raster format at the pixel frequency as illustrated in FIG. 10.
FIG. 10 illustrates a typical prior art laser printer subsystem without subpixel modulation wherein the printer controller shift register 10 is triggered by the pixel clock on lead 12 to apply the video signal to modulate laser diode 26 on and off.
One known digital modulation technique provides gating a pixel into m sub-pixels along the direction of the laser scan, such that up to (nxm)+1 grays are possible. For instance for m=8, the possible grays between the pixels of FIGS. 2 and 3 are illustrated in FIGS. 11 through 19.
The width and position of a pixel are controlled to 1/m of a pixel. This can be implemented by clocking the data to the engine at m times the pixel rate as shown in FIG. 20. This allows m incremental gray values between each of the steps in FIGS. 1-9. FIGS. 11-19 show how the single step from FIG. 2 to FIG. 3 is refined into multiple gray steps.
Thus, more gray gradiations can be accomplished by increasing the number of subpixel slices. In practice, there is a limit on how fine one can gate the subpixel slices. Due to inevitable noise pickup between circuit modules, there is a limit on how well the position and the slew rate of pulse edges can be controlled in a consistent and unvarying fashion. Another limiting factor is how fast the switching rate needs to be increased to before such rates become impractical with today's circuit components and methodology.
In a typical analog modulation technique, a free running, fixed frequency triangular waveform is generated by the modulator circuit. As illustrated in FIG. 21, a digitally encoded gray value is converted to an analog signal representing the said gray value by passing it through a digital-to-analog converter. This resulting analog signal is compared with the aforementioned triangular waveform. The bilevel modulating signal to the laser diode is derived by comparing the said analog gray signal to the said triangular wave signal. With existing implementations, the triangular wave signal's period can span multiple pixels; this allows the necessary gray gradiations to be realized without over stressing the capabilities of the electrophotography process. The sawtooth waveform and resultant variable width pulse signal are illustrated in FIGS. 22 and 23, respectively. In some implementations, the width of the triangular waveform spans the width of several pixels to allow more gray levels.
This encoded scheme allows up to 2.sup.p levels of gray per pixel, eliminating the bandwidth problems with the decoded digital approach. However, note that each subpixel necessarily grows from the pixel's center. As previously mentioned, this means there is no way to control a subpixel's position and such analog scheme therefore does not lend itself to the aforementioned techniques of enhancing image halftones and the apparent resolution of text and graphics.